A urea after-treatment technology is a method of reducing NOx using a reaction formula of 2NO+2CO (or HC)→N2+2CO by which urea is transformed into ammonia (NH3), and then, the ammonia reacts selectively with NOx through a selective catalytic reduction (SCR) catalyst. The urea after-treatment technology includes a urea tank storing ammonia as the urea which is harmless to humans, a SCR, and an injector for injecting the urea into the SCR.
In recent years, since the exhaust emission standards (for Bin 5) of North America restrict the amount of NOx emission in exhaust gas and particularly restrict vehicles from being driven during exhaustion of urea in the urea tank, it is necessary to periodically replenish the urea.
Typically, replenishment of urea in the urea tank is performed by opening a urea injection port located inside the tank using a trunk room or by opening a urea injection port located outside along with a fuel injection port located. In addition, urea is injected into the urea tank at high speed using a urea injection gun or is injected using a separate urea container.
However, when a membrane valve having a small ventilation rate is applied to the urea tank, the membrane valve has a limit to endure a pressure increase in the urea tank caused by injection of urea. Structural weakness of the membrane valve causes an overflow phenomenon in which urea flowing backward at the end of injection is spouted from the urea injection port via a leveling pipe, resulting in dissatisfaction of a user on injection or replenishment of urea.
When the urea injection port is particularly provided in the trunk room, the overflow of urea through the urea injection port causes contamination of the trunk room, metal corrosion, and ammoniac smell inside the vehicle.
In addition, the urea tank has a relatively small capacity of 25˜30 L to 15˜20 L on the ground of a reduction in weight and an improvement in layout. However, the small capacity of the urea tank in turn causes urea to be frequently injected. For this reason, it is very important to prevent the urea overflow generated through the urea injection port.
For the prevention of the urea overflow, a roll over valve (ROV) may be applied to a fuel tank, instead of the membrane valve. However, the application of the ROV causes corrosion of urea and crystallization of urea during exposure thereof to the atmosphere. Accordingly, it is difficult to apply the ROV.